This invention relates to a storage system for storing data, and more particularly to a technique that uses a nonvolatile semiconductor memory capable of erasing data each block, as a storage medium.
In recent years, a storage system for storing data safely for a long time has been required. For example, it is required by law that a financial institution, a medical institution, and the like have their document data stored in a write-once memory in which data cannot be erased or rewritten.
The storage system generally has a random-accessible nonvolatile storage medium. Examples of the random-accessible nonvolatile storage medium include a magnetic disk and an optical disk. Also, the storage system that is currently in the mainstream has a large number of small disk drives.
Also, with the advancement of a semiconductor technology, a flash erasable nonvolatile semiconductor memory has been developed. An example of the flash erasable nonvolatile semiconductor memory includes a flash memory. The storage system that uses the flash memory as the storage medium is superior to the storage system, which has a large number of the small disk drives, in terms of useful life, electric power saving, access time and the like.
Here, the flash memory will be explained. In the flash memory, according to its features, the data cannot be directly rewritten. In other words, in the flash memory, the stored valid data must be saved to rewrite the stored data. Next, the stored data is erased by the block. Then, the data is written to the block where the data is erased. It should be noted that the block is a storage area as unit by which the data is flash-erased.
Specifically, in the flash memory, ‘1’ can be rewritten to ‘0’. However, ‘0’ cannot be rewritten to ‘1’. So, in the flash memory, the entire block is erased to rewrite the data.
In the flash memory, it takes longer time to write and erase the data than to read the data. For this reason, in the flash memory, in order to rewrite the data, the data must be erased and written continuously. Thus, the operation becomes desperately slow.
A storage system for solving those problems is disclosed in JP 05-27924 A. The storage system of the prior art introduces an address control method of giving flexibility to an address relation between a host CPU and an external memory so that a physical address of the semiconductor memory is not unilaterally determined by a logical address possessed by a command of the host CPU.